There is known an apparatus for measuring a cornea shape manually, in which predetermined point source, projected onto a cornea and prisms, etc., are disposed within an optical path of an observing optical system are moved as well as rotated while observing the corneal reflection images of the point sources, so that the images are in a predetermined condition. The radius of curvature of the cornea is measured based on the amount of movement as well as the amount of rotation thus obtained.
Recently photoelectric type apparatuses for measuring a corneal shape are widely utilized in lieu of the conventional manual measuring apparatus. Various measuring methods of the photoelectric type measuring apparatus are proposed. Common to these methods are the following features. A set of point sources which are substantially on a same circle (including a ring aperture) is projected onto a cornea. The corneal reflection images of the point sources are detected by means of one or a plurality of one-dimentional detecting means or two-dimensional detecting means. Supposing that the corneal reflection images detected by the detecting means are on a single ellipse the corneal shape is calculated on the basis thereof (subjecting it to operations such as connections, etc.).
The applicants of the present invention have also disclosed, in JP-B-Hei 1-19896, an apparatus for measuring a corneal shape comprising: 1 projecting means for projecting point sources including 2 point sources which are placed in symmetry with respect to the optical axis of the measuring optical system and at least another one point source which is placed on a same circle as that of the 2 point sources onto a cornea; 2 detecting means for detecting the point sources reflected by the cornea; 3 an imaging optical system which focuses the corneal reflection images of the point sources at least in the direction detected by the detecting means; 4 calculating means which calculates the coordinates of the center of the ellipse containing the images of the point sources from the detected positions of the 2 points which are symmetric with respect to the optical axis of the measuring optical system, and then calculates the shape of the ellipse containing the images of the point sources by calculating coordinates of the center and the coordinates of the 3 points in the corneal reflection image.
The photoelectric-type apparatus for measuring a corneal shape described above has a merit that is available to measure a corneal shape automatically independent of skills of the examiner. Although in the above-described measuring method according to the prior art, a corneal shape is measured presuming that a cornea has a uniform toric surface however, a cornea does not necessarily have a uniform toric surface. Therefore an apparatus having the construction as described above has a problem in that a corneal shape is measured and displayed as if it had a uniform toric surface.